Dissolved Reactive P Load Research Articles

Tillage and cover cropping influence phosphorus dynamics in soil and water pools

AbstractWinter cover crops (CCs) have the potential to reduce phosphorus (P) loss by temporarily fixing P into CC biomass. A field experiment with no‐tillage (NT) and conventional tillage (CT) was used to study the ability of different CC species planted after corn (Zea mays L.) and soybean (Glycine max L.) harvests to reduce the P availability in soil solution. The effect of three crop rotations (corn–no CC–soybean–no CC [C–S], corn–cereal rye (Secale cereale)–soybean–hairy vetch (Vicia villosa) [C–R–S–HV], corn–cereal rye–soybean–oats (Avena sativa)+ radish (Raphanus sativus L.) [C–R–S–OR]) and two tillage (NT and CT) treatments was determined on soil available P and soil solution P content through pan (A horizon) and tension (100‐cm depth) cup lysimeters. The experiment was set up as a randomized complete block design with tillage as a split factor with three replicates. Over the study period, incorporating hairy vetch in C–R–S–HV rotation reduced the Mehlich‐3 P content in soil by 26%–29% compared to the C–S and C–R–S–OR rotation. Both CC rotations (C–R–S–HV and C–R–S–OR) were effective in reducing dissolved reactive P (DRP) concentration in pan and tension cup lysimeters compared to the C–S in both CT and NT systems. However, these results varied with CC species grown and seasonal variability in precipitation. A significantly lower DRP load with crop rotation and tillage treatments was observed mainly during the CC growing season. During the study period, crop rotations with reduced labile soil P content and DRP loss were ranked in an order of C–R–S–HV > C–R–S–OR > C–S. Overall, this study showed that CCs have the potential in both CT and NT systems to significantly reduce P in soil and soil solution, and these effects are resilient to a wide range of precipitation conditions.

Comparison of manure application methods on nutrient and metal loss to snowmelt

Manure injection is well documented to reduce rain-driven nutrient losses, but the effect on snowmelt-driven losses is less studied. This study examined the surface application versus injection of liquid swine manure on phosphorus (P), nitrate‑nitrogen (nitrate-N), zinc (Zn), manganese (Mn), iron (Fe), magnesium (Mg), and calcium (Ca) release from soils to snowmelt. Manure was applied in the fall of 2021 to four replicated plots of surface-applied or injected treatments, and boxes were installed to collect snowmelt in each plot, including four unmanured (control) plots. In the spring of 2022, the snowmelt was collected from each box for 10 days, volume was recorded, and analyzed for dissolved reactive P (DRP), nitrate-N, Zn, Mn, Fe, Mg, and Ca concentrations, and pH. The mean daily snowmelt volume and snowmelt depth ranged from 2.9 to 25.2 L, and 2.7 to 23.3 mm, respectively. The mean snowmelt DRP concentrations for manure injected, surface-applied, and control treatments were 0.66 ± 0.07, 0.89 ± 0.07, and 0.65 ± 0.07 mg L−1, respectively. The mean DRP concentration did not vary among the manure application methods for the first 5 days of snowmelt. However, DRP was significantly higher in the manure surface-applied treatment, compared to both control and manure-injected treatments on days 7 and 8, compared to the control on day 6, and compared to the manure-injected on day 9. The effect of the manure application method on nitrate-N (3.27 to 3.60 mg L−1) and metal concentrations in the snowmelt was not significant. The cumulative loads (mean across application methods and sampling days in kg ha−1) of DRP (0.56), nitrate-N (4.03), and metals [Zn (0.11), Mn (0.01), Fe (0.18), Mg (8.03), Ca (21.70)] mobilized with snowmelt flooding did not significantly vary among the manure application methods. Results suggest that DRP, nitrate, and metal loads were primarily controlled by the snowmelt volume rather than their concentrations. Therefore, land management practices that reduce the volume of snowmelt discharge could be more effective for reducing the P, nitrate-N, and metal loss to snowmelt from manured soils.

A New Approach to Predict Tributary Phosphorus Loads Using Machine Learning- and Physics-Based Modeling Systems.

Tributary phosphorus (P) loads are one of the main drivers of eutrophication problems in freshwater lakes. Being able to predict P loads can aid in understanding subsequent load patterns and elucidate potential degraded water quality conditions in downstream surface waters. We demonstrate the development and performance of an integrated multimedia modeling system that uses machine learning (ML) to assess and predict monthly total P (TP) and dissolved reactive P (DRP) loads. Meteorological variables from the Weather Research and Forecasting (WRF) Model, hydrologic variables from the Variable Infiltration Capacity model, and agricultural management practice variables from the Environmental Policy Integrated Climate agroecosystem model are utilized to train the ML models to predict P loads. Our study presents a new modeling methodology using as testbeds the Maumee, Sandusky, Portage, and Raisin watersheds, which discharge into Lake Erie and contribute to significant P loads to the lake. Two models were built, one for TP loads using 10 environmental variables and one for DRP loads using nine environmental variables. Both models ranked streamflow as the most important predictive variable. In comparison with observations, TP and DRP loads were predicted very well temporally and spatially. Modeling results of TP loads are within the ranges of those obtained from other studies and on some occasions more accurate. Modeling results of DRP loads exceed performance measures from other studies. We explore the ability of both ML-based models to further improve as more data become available over time. This integrated multimedia approach is recommended for studying other freshwater systems and water quality variables using available decadal data from physics-based model simulations.

Evaluating fall application of soil amendments to mitigate phosphorus losses during spring snowmelt

The potential of soil amendments in reducing phosphorus (P) loss has been reported under simulated snowmelt, but not evaluated in the field, under snowmelt conditions. We examined the effectiveness of alum (Al2(SO4)3·18H2O), gypsum (CaSO4·2H2O), and Epsom salt (MgSO4·7H2O) in reducing P loss from two flood-prone agricultural fields (non-manured field with initial soil test P concentrations, STP, of 76 mg kg−1 and manured field with initial STP of 202 mg kg−1), during snowmelt on Canadian Prairies. Amendments were applied (2.5 Mg ha−1) in 2020 fall to four replicated plots (3 m × 1 m) with pre-installed runoff boxes (1.2 m × 0.9 m). In the 2021 spring, snowmelt water in each runoff box was pumped out, volume recorded for each sampling day, and analysed for dissolved reactive P (DRP). Snowmelt DRP concentrations was greater in manured soil (0.73 ± 0.23 mg L−1) with high STP, than in non-manured soil (0.28 ± 0.10 mg L−1.) with low STP, implying a greater risk of P loss to snowmelt runoff when STP is high. Cumulative snowmelt DRP loads in non-manured soil ranged from 3.1 to 3.8 mg (equivalent to 0.03–0.04 kg ha−1) and were not significantly different among treatments. In the control treatment of the manured soil, the cumulative snowmelt DRP load from runoff boxes was 20.8 mg (equivalent to 0.19 kg ha−1), while amended treatments had 42 to 68 % lower cumulative DRP load than control, with the difference being significant only in Epsom salt treatment. Cumulative snowmelt DRP load was significantly and positively correlated to cumulative snowmelt volume (R2 = 0.84 for each soil). This study shows the potential of fall application of soil amendments, particularly Epsom salt, to reduce P loss to snowmelt from soils with high STP.

Assessing the concept of control points for dissolved reactive phosphorus losses in subsurface drainage.

Agricultural phosphorus (P) loss, which is highly variable in space and time, has been studied using the hot spot/hot moment concept, but increasing the rigor of these assessments through a relatively newer "ecosystem control point" framework may help better target management practices that provide a disproportionate water quality benefit. Sixteen relatively large (0.85ha) subsurface drainage plots in Illinois were used as individual observational units to assess dissolved reactive P (DRP) concentrations and losses within a given field over four study years. Three plot-months were identified as DRP control points (one export and two transport control points), where each plot-month contributed>10% of the annual DRP load from the field. These control points occurred on separate plots and in both the growing and nongrowing seasons but were likely related to agronomic P applications. Elevated soil test P, especially near a historic farmstead, and soil clay content were spatial drivers of P loss across the field. The nongrowing season was hypothesized to be the most significant period of P loss, but this was only documented in two of the four study years. A cereal rye (Secale cereale L.) cover crop did not significantly reduce DRP loss in any year, but there was also no evidence of increased drainage P losses due to freezing and thawing of the cover crop biomass. This work confirmed annual subsurface drainage DRP losses were agronomically small(<3% of P application rate), although the range of DRP concentrations relative to eutrophication criteria still demonstrated a potential for negative environmental impact. The control point concept may provide a new lens to view drainage DRP losses, but this framework should be refined through additional within-field studies because mechanisms of P export at this field were more nuanced than just the presence of tile drainage (i.e., a transport control point).

Cover crops differentially influenced nitrogen and phosphorus loss in tile drainage and surface runoff from agricultural fields in Ohio, USA

Nitrogen (N) and phosphorus (P) loss from crop production agriculture is transported to adjacent and downstream water bodies, resulting in negative environmental impacts including harmful and nuisance algal blooms. Cover crops are a conservation management practice that replaces bare soil with vegetation outside of the cash crop growing season, purportedly reducing N and P loss by increasing water and nutrient demand in agroecosystems. In this study, we compared nitrate (NO3−-N), total N (TN), dissolved reactive P (DRP), and total P (TP) loads in subsurface (tile) drainage and surface runoff from fields with cover crop management (CC) and fields without cover crop management (NoCC) using continuous monitoring data from 40 agricultural fields located throughout northcentral Ohio, United States (US). We found that average monthly tile NO3−-N and TN loads from CC fields were ~50% less than NoCC fields, while average monthly tile discharge, DRP, and TP loads did not differ between CC and NoCC fields. Cover crops also did not significantly influence average monthly surface metrics. Cover crops reduced monthly totals of tile NO3−-N and TN loads by ~1.0–2.6 kg N ha−1 from January to June (winter and spring), coinciding with critical periods of nutrient loss from agroecosystems in the midwestern US, but increased monthly totals of tile DRP (by 0.4–12.1 g DRP ha−1) and TP (by 1.2–31.6 g TP ha−1) loads during some months. We found similar patterns at the annual time scale whereby CC fields had lesser cumulative annual totals of tile NO3−-N and TN but greater cumulative annual totals of tile DRP and TP. These results show that the influence of cover crops on N loads, but not P, were consistent across temporal scales of examination, demonstrating that cover crops effectively increased N demand and mitigated N losses from agricultural fields. The variable influence of cover crops on P loads underscores the need for greater understanding of the factors and mechanisms that control P loss in systems that include cover crop management. Furthermore, these findings stress the importance of identifying and selecting conservation management practices tailored to the natural resource concern.

Less Agricultural Phosphorus Applied in 2019 Led to Less Dissolved Phosphorus Transported to Lake Erie.

Extreme precipitation events affect water quantity and quality in various regions of the world. Heavy precipitation in 2019 resulted in a record high area of unplanted agricultural fields in the U.S. and especially in the Maumee River Watershed (MRW). March-July phosphorus (P) loads from the MRW drive harmful algal bloom (HAB) severity in Lake Erie; hence changes in management that influence P export can ultimately affect HAB severity. In this study, we found that the 2019 dissolved reactive P (DRP) load from March-July was 29% lower than predicted, while the particulate P (PP) load was similar to the predicted value. Furthermore, the reduced DRP load resulted in a less severe HAB than predicted based on discharge volume. The 29% reduction in DRP loss in the MRW occurred with a 62% reduction in applied P, emphasizing the strong influence of recently applied P and subsequent incidental P losses on watershed P loading. Other possible contributing factors to this reduced load include lower precipitation intensity, altered tillage practices, and effects of fallow soils, but more data is needed to assess their importance. We recommend conservation practices focusing on P application techniques and timing and improving resiliency against extreme precipitation events.

Controls on subsurface nitrate and dissolved reactive phosphorus losses from agricultural fields during precipitation-driven events

The magnitude of nitrogen (N) and phosphorus (P) exported from agricultural fields via subsurface tile drainage systems is determined by site-specific interactions between weather, soil, field, and management characteristics. Here, we used multiple regression analyses to evaluate the influence of 29 controls of precipitation event-driven discharge, nitrate (NO3−-N) load, and dissolved reactive P (DRP) load from subsurface tile drains, leveraging a unique dataset of ~7000 precipitation events observed across 40 agricultural fields (n = 190 site years) instrumented to collect continuous water quality samples. We calculated marginal effects of significant controls and assessed the modifying influence of event rainfall, duration, and intensity, and antecedent precipitation. Tile discharge was strongly and positively influenced by previous 7-day precipitation and total rainfall and negatively influenced by daily temperature and tile spacing. Both tile NO3−-N and DRP loads were positively influenced by transport and source variables, including event discharge and total fertilizer applied as well as soil test P (STP) in the case of tile DRP load; factors with the strongest negative influence on tile NO3−-N and DRP loads were related to time of year. The strength and direction of both positive and negative controls also varied with precipitation characteristics. For example, the positive influence of event discharge on nutrient loads lessened as event duration, event intensity, and previous 7-day precipitation increased, while the positive influence of N and P sources strengthened, particularly in response to extreme (or maximum) events. Results here demonstrate the predominant role of transport and source controls while accounting for interactive effects among site-specific characteristics and underscore the importance of storm dynamics when managing N and P loss from agricultural fields.

Closed depressions and soil phosphorus influence subsurface phosphorus losses in a tile-drained field in Illinois.

Artificial subsurface (tile) drainage systems can convey phosphorus (P) from agricultural fields to surface waters; however, controls of subsurface dissolved reactive P (DRP) losses at the sub-field scale are not fully understood. We characterized subsurface DRP loads and flow-weighted mean concentration (FWMC) from January 2015 through September 2017 to determine seasonal (growing vs. non-growing) patterns from 36 individually monitored plots across a farm under a corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotation in east-central Illinois. Using linear mixed models, we investigated the effects of soil test P (STP), depression depth, and their interaction with precipitation and P fertilization on subsurface DRP losses. Dissolved reactive P loads in drainage tiles increased with precipitation and were greatest during the non-growing season (NGS) in 2016 and 2017. Annual subsurface DRP loads were positively related to STP, and during the NGS, there was a positive relationship between depression depth quantified at the plot-scale and subsurface DRP loads and FWMC. Along a depression-depth gradient, piecewise regression displayed a threshold at a depth of 0.38m at which STP increased, indicating soil P accumulation in deeper closed depressions. Our study highlights the need to identify areas with the greatest risk of subsurface P losses to implement sub-field scale nutrient management practices.

Runoff water quality after low-disturbance manure application in an alfalfa-grass hay crop forage system.

The impacts of low-disturbance manure application (LDMA) on runoff water quality in hay crop forages are not well known. Our objective in this study was to determine surface runoff losses of total nitrogen (TN), ammonium N (NH4 -N), nitrate N (NO3 -N), total phosphorus (TP), dissolved reactive P (DRP), and suspended sediment from alfalfa (Medicago sativa L.)-grass plots in central Wisconsin after surface broadcasting manure and LDMA compared with no application. Treatments were (a) surface banding (BAND), (b) surface banding with aeration (A/B), (c) shallow disk injection (INJECT), (d) surface broadcast (BCAST), and (e) a no-manure control (CONT). Runoff events were generated (n=7) from replicated plots following a standardized rainfall simulation protocol. Although runoff was variable across plots and within treatments, mean runoff concentrations of TN (P=.03), NH4 -N (P=.03), TP (P=.001), and DRP (P<.0001) were lower for incorporated (INJECT and A/B) vs. unincorporated (BCAST and BAND) treatments. INJECT had lower mean DRP concentration (P=.02) than A/B and was similar to CONT and had lower cumulative TN (P=.05), TP (P=.07), and DRP (P=.01) loads than A/B. Additionally, TP, TN, DRP, and NH4 -N loads and concentrations were strongly related with soil surface manure coverage extent (R2 = 0.50-0.84; P<.0001), suggesting that manure was a main source of N and P losses. Although INJECT appeared to be the most effective in mitigating nutrient loss in surface runoff, more research is needed to determine LDMA impacts on farm economics, soil properties, and runoff water quality.

Solid Cattle Manure Less Prone to Phosphorus Loss in Tile Drainage Water.

Forms (e.g., liquid and solid) of manure influence the risk of P loss after land application. The objective of this study was to investigate the effects of P-based application of various forms of cattle manure (liquid, LCM; or solid, SCM) or inorganic P as triple superphosphate (IP) on soil P losses in tile drainage water. A 4-yr field experiment was conducted in a clay loam soil with a corn ( L.)-soybean [ (L.) Merr.] rotation in the Lake Erie basin. Over the 4 yr, the dissolved reactive P (DRP) flow-weighted mean concentration (FWMC) in tile drainage water was greater under SCM fertilization than under either IP or LCM fertilization. Despite its lower value on an annual basis, DRP FWMC rose dramatically immediately after LCM application. However, the differences in DRP FWMC did not result in detectable differences in DRP loads. Regarding particulate P and total P losses during the 4 yr, they were 68 and 47%, respectively, lower in the soils amended with SCM than in those with IP, whereas both values were similar between IP and LCM treatments. Overall, the P contained in solid cattle manure was less prone to P loss after land application. Accordingly, the present results can provide a basis for manure storage and application of best management practices designed to reduce P losses and improve crop growth.

Phosphorus, Iron, and Aluminum Losses in Runoff from a Rotationally Grazed Pasture in Georgia

Abstract. Well-managed grazing systems can provide valuable ecosystem services, such as reducing sediment and phosphorus (P) loading to nearby waterways. However, the available long-term data to fully support this hypothesis are limited. In this article, we describe flow-weighted concentrations (FWCs) and loads for dissolved reactive P (DRP), total P (TP), iron (Fe), and aluminum (Al) over 11 years (1999-2009) from a 7.8 ha rotationally grazed pasture (W1) near Watkinsville, Georgia. The region is characterized by Fe and Al rich and acidic Ultisols. Cattle numbering 21 to 224 (mean 91) grazed W1 on 69 occasions for 1 to 71 d (mean 19.2). Of 74 runoff events, 20 occurred when the monthly rainfall was below the long-term average (deficit period) and 54 occurred during non-deficit periods. Samples were collected from 43 of 74 runoff events for nutrient analyses. Event FWC (mg L-1) ranged from 0.38 to 7.07 for DRP (mean 1.91), from 0.36 to 7.60 for TP (mean 2.43), from 0.03 to 0.55 for Fe (mean 0.23), and from 0.43 to 553 µg L-1 for Al (mean 65 µg L-1). Event load (kg ha-1) ranged from 0.00 to 0.45 for DRP (mean 0.10), from 0.00 to 0.55 for TP (mean 0.12), from 0.00 to 0.11 for Fe (mean 0.02), and from 0.00 to 0.10 for Al (mean 0.01). The total load (kg ha-1) was 4.12 for DRP, 5.12 for TP, 0.71 for Fe, and 0.25 for Al. DRP accounted for 80% of the TP FWC and load. Cattle presence increased sediment load, but the difference was not statistically significant. There was high correlation between Fe and DRP loads (r = 0.87), a likely indicator of erosion-induced losses due to cattle treading. Cattle presence increased FWCs but not loads for DRP and TP. The FWCs for DRP and TP were not different between deficit and non-deficit periods, but mean loads were 3-fold to 4-fold greater during non-deficit periods. Means from the six largest P loss events were 3-fold greater for FWC and 7-fold greater for load than the remaining 37 events. These six large events accounted for 53% of the total P load. Less than 1% of the inorganic P applied and redeposited through manure was lost in runoff. The study demonstrated that hydrologic transport processes were the dominant drivers of pollutant fluxes and highlighted the possible mitigation of pollutant fluxes through grazing management that includes maintenance of good grass cover, effective rotational grazing, and limited fertilization. Keywords: Calving, Cattle, Dissolved reactive phosphorus, Drought, Eutrophication, Manure, Runoff, Total phosphorus, Water quality.

Decreasing Phosphorus Loss in Tile-Drained Landscapes Using Flue Gas Desulfurization Gypsum.

Elevated phosphorus (P) loading from agricultural nonpoint-source pollution continues to impair inland waterbodies throughout the world. The application of flue gas desulfurization (FGD) gypsum to agricultural fields has been suggested to decrease P loading because of its high calcium content and P sorbing potential. A before-after control-impact paired field experiment was used to examine the water quality effects of successive FGD gypsum applications (2.24 Mg ha; 1 ton acre each) to an Ohio field with high soil test P levels (>480 ppm Mehlich-3 P). Analysis of covariance was used to compare event discharge, dissolved reactive P (DRP), and total P (TP) concentrations and loadings in surface runoff and tile discharge between the baseline period (86 precipitation events) and Treatment Period 1 (42 precipitation events) and Treatment Period 2 (84 precipitation events). Results showed that, after the first application of FGD gypsum, event mean DRP and TP concentrations in treatment field tile water were significantly reduced by 21 and 10%, respectively, and DRP concentrations in surface runoff were significantly reduced by 14%; however, no significant reductions were noted in DRP or TP loading. After the second application, DRP and TP loads were significantly reduced in surface runoff (DRP, 41%; TP 40%), tile discharge (DRP, 35%; TP, 15%), and combined (surface + tile) discharge (DRP, 36%; TP, 38%). These findings indicate that surface application of FGD gypsum can be used as a tool to address elevated P concentrations and loadings in drainage waters.

Effect of crop type and season on nutrient leaching to tile drainage under a corn-soybean rotation

Subsurface tile drainage is a significant pathway for nitrogen (N) and phosphorus (P) transport from agricultural fields. The objective of this study was to evaluate N and P loss through tile drainage under corn (Zea mays L.) and soybean (Glycine max L.) production in a corn–soybean rotation typical of agricultural management across the eastern Corn Belt of the US Midwest. Differences in nutrient concentrations and loadings between crop type and between growing (GS) and nongrowing seasons (NGS) were assessed. From 2005 through 2012, discharge and water quality were monitored at three end-of-tile locations that had estimated contributing areas ranging from 7.7 to 14.9 ha (19.0 to 36.8 ac) in a headwater watershed in central Ohio, United States. Nitrate-N (NO3-N) and dissolved reactive P (DRP) were the primary (>75%) forms of N and P in drainage water. DRP concentration and loading was not significantly different between crop types, but differed significantly by season. Mean weekly DRP concentration (0.22 mg L−1 [0.22 ppm]) was greater during the GS, while mean weekly DRP load (0.010 kg ha−1 [0.009 lb ac−1]) was greater in the NGS. In comparison, NO3-N concentration and load was dependent on the interaction between crop type and season, with the greatest NO3-N concentration (17.1 mg L−1) observed during the GS under corn production. Differences in N and P loss to tile drains were attributed to the timing of nutrient application and differences in seasonal discharge. Practices such as cover crops and drainage water management that target nutrient transport in the NGS should be explored as a means to decrease annual N and P loads. Adherence to recommended 4R nutrient stewardship (right fertilizer source, right rate, right time, and right placement) practices should also help minimize nutrient leaching to tile drains under a corn–soybean rotation.

Characterizing phosphorus dynamics in tile-drained agricultural fields of eastern Wisconsin

Summary Artificial subsurface drainage provides an avenue for the rapid transfer of phosphorus (P) from agricultural fields to surface waters. This is of particular interest in eastern Wisconsin, where there is a concentrated population of dairy farms and high clay content soils prone to macropore development. Through collaboration with private landowners, surface and tile drainage was measured and analyzed for dissolved reactive P (DRP) and total P (TP) losses at four field sites in eastern Wisconsin between 2005 and 2009. These sites, which received frequent manure applications, represent a range of crop management practices which include: two chisel plowed corn fields (CP1, CP2), a no-till corn–soybean field (NT), and a grazed pasture (GP). Subsurface drainage was the dominant pathway of water loss at each site accounting for 66–96% of total water discharge. Average annual flow-weighted (FW) TP concentrations were 0.88, 0.57, 0.21, and 1.32 mg L −1 for sites CP1, CP2, NT, and GP, respectively. Low TP concentrations at the NT site were due to tile drain interception of groundwater flow where large volumes of tile drainage water diluted the FW-TP concentrations. Subsurface pathways contributed between 17% and 41% of the TP loss across sites. On a drainage event basis, total drainage explained between 36% and 72% of the event DRP loads across CP1, CP2, and GP; there was no relationship between event drainflow and event DRP load at the NT site. Manure applications did not consistently increase P concentrations in drainflow, but annual FW-P concentrations were greater in years receiving manure applications compared to years without manure application. Based on these field measures, P losses from tile drainage must be integrated into field level P budgets and P loss calculations on heavily manured soils, while also acknowledging the unique drainage patterns observed in eastern Wisconsin.

Nutrient Leaching during Establishment of Simulated Residential Landscapes

Research evaluating nutrient losses during the establishment of plant material in mixed residential landscapes is limited. The objectives of this study were to determine the effect of vegetative cover type, compost application, and tillage on nutrient losses during the establishment of landscape plants. Twenty-four small plots constructed with subsoil fill were planted with St. Augustinegrass [ (Walter) Kuntze] and mixed ornamental species in a randomized complete block design. Plots received composted dairy manure solids at a rate of 0 or 50.8 m ha- in combination with shallow tillage or aeration. Cumulative leachate loads and flow-weighted mean concentrations of NH-N, NO + NO-N, and dissolved reactive P (DRP) were calculated periodically and annually to assess nutrient leaching from landscape plots. Higher cumulative leachate volume, inorganic N and DRP loads, and mean NO + NO-N and DRP concentrations were observed under ornamental cover during one or more study periods, which we attribute to differences in root density and shoot biomass between mixed ornamental species and turfgrass during establishment. Greater cumulative leachate inorganic N loads were reported from composted soils than from unamended soils or soils receiving only tillage or aeration. Inorganic N and DRP loads were similar in magnitude to reported leaching losses from agricultural systems. Better management of nutrients and water in woody ornamental plant beds during plant establishment is needed due to differences in plant growth habits compared with turfgrass. Nutrient content of organic amendments should be considered when applying these materials as a soil conditioner in new residential landscapes.

Phosphorus and faecal bacteria in runoff from horse paddocks and their mitigation by the addition of P-sorbing materials

The growing popularity of horse keeping is accompanied by an increase of phosphorus (P) and faecal micro-organisms from outdoor paddocks. We used an indoor rainfall simulation to monitor concentrations of dissolved reactive P (DRP) and faecal coliforms in runoff and percolation water from different paddock footings. Drainage water was also monitored from two paddocks constructed of woodchips. Sand retained more DRP (p&lt;0.0001) and coliforms from percolation water than woodchips. Some of the footings were amended with P-sorbing materials, such as [Ca(OH)2], [Fe2(SO4)3], or Fe-gypsum, to retain DRP. High DRP concentrations (17–18 mg l-1) were observed in runoff from a woodchip footing amended earlier with Ca(OH)2 and in sand footing amended with CaCO3. However, application of Fe-gypsum to woodchips decreased the DRP load in percolation water by 83% compared to the footing without Fe-gypsum. Fe compounds were better than Ca compounds. The decrease in coliforms was usually small due to the modest pH changes in the water.

Dairy Heifer Manure Management, Dietary Phosphorus, and Soil Test P Effects on Runoff Phosphorus

Manure application to cropland can contribute to runoff losses of P and eutrophication of surface waters. We conducted a series of three rainfall simulation experiments to assess the effects of dairy heifer dietary P, manure application method, application rate, and soil test P on runoff P losses from two successive simulated rainfall events. Bedded manure (18-21% solids) from dairy heifers fed diets with or without supplemental P was applied on a silt loam soil packed into 1- by 0.2-m sheet metal pans. Manure was either surface-applied or incorporated (Experiment 1) or surface-applied at two rates (Experiment 2) to supply 26 to 63 kg P ha. Experiment 3 evaluated runoff P from four similar nonmanured soils with average Bray P1-extractable P levels of 11, 29, 51, and 75 mg kg. We measured runoff quantity, total P (TP), dissolved reactive P (DRP), and total and volatile solids in runoff collected for 30 min after runoff initiation from two simulated rain events (70 mm h) 3 or 4 d apart. Manure incorporation reduced TP and DRP concentrations and load by 85 to 90% compared with surface application. Doubling the manure rate increased runoff DRP and TP concentrations an average of 36%. In the same experiment, P diet supplementation increased water-extractable P in manure by 100% and increased runoff DRP concentration threefold. Concentrations of solids, TP, and DRP in runoff from Rain 2 were 25 to 75% lower than from Rain 1 in Experiments 1 and 2. Runoff DRP from nonmanured soils increased quadratically with increasing soil test P. These results show that large reductions in P runoff losses can be achieved by incorporation of manure, avoiding unnecessary diet P supplementation, limiting manure application rate, and managing soils to prevent excessive soil test P levels.

Impact of Manure Phosphorus Fractions on Phosphorus Loss from Manured Soils after Incubation

The risk of P loss from manured soils is more related to P fractions than total P concentration in manure. This study examined the impact of manure P fractions on P losses from liquid swine manure- (LSM), solid cattle manure- (SCM), and monoammonium phosphate- (MAP) treated soils. Manure or fertilizer was applied at 50 mg P kg soil, mixed, and incubated at 20°C for 6 wk to simulate the interaction between applied P and soil when P is applied well in advance of a high risk period for runoff. Phosphorus fractions in manure were determined using the modified Hedley fractionation scheme. We used simulated rainfall (75 mm h⁻¹ for 1 h) to quantify P losses in runoff from two soils (sand and clay loam). The proportion of total labile P (total P in water+NaHCO fractions) in manure was significantly greater in LSM (70%) than SCM (44%). Mean dissolved reactive P (DRP) load in runoff over 60 min was greatest from MAP-treated soil (18.1 mg tray⁻¹), followed by LSM- (14.0 mg tray⁻¹) and SCM- (11.0 mg tray⁻¹) treated soils, all of which were greater than mean DRP load from the check (5.2 mg tray⁻¹). Total labile P (water+NaHCO) in manure was a more accurate predictor of runoff DRP loads than water extractable P, alone, for these two soils. Therefore, NaHCO extraction of manure P may be a useful tool for managing the risk of manure P runoff losses when manure is applied outside a high risk period for runoff loss.

Phosphorus source areas in a dairy catchment in Otago, New Zealand

It is important to recognise source areas of phosphorus (P) in agricultural catchments and to understand how they contribute to catchment losses of P in order to effectively target mitigation strategies to decrease losses to surface waters. In a small dairy catchment (4.1 ha), soil physical properties and overland flow from pasture, a laneway, and around a watering trough were measured, together with subsurface flows from pasture and catchment discharge. Soil measured around the trough and in the laneway was found to be enriched in Olsen P (56 and 201 mg P/kg, respectively) compared with the pasture (24 mg P/kg), as well as having a greater bulk density resulting from more frequent use by animals. Dissolved P losses from lane and trough plots were greatly enhanced via dung. At the catchment scale, sources and transport processes resulted in losses mainly in the particulate P form (0.21 mg/L), while dissolved reactive P (DRP) concentrations were enriched during storm events (0.08 mg/L). Subsurface flow was found to be an important contributor of discharge and likely P losses, and this warrants further investigation. The scaling up of overland-flow plot data suggested that the laneway contributed up to 89% of the DRP load when surface overland flow was likely. This represents a substantial source of P loss on dairy farms. Additionally, the variation of sources and transport processes with season adds another aspect to the critical source area concept, and suggests that given the loss during summer and high algal availability of dissolved P, mitigation strategies should target decreasing dissolved P loss from the laneway.